Use of momentum transfer actuators for motion control of flexible mechanical structures

ABSTRACT

This invention discloses a method for controlling the position of flexible portions of a mechanical structure by attaching one or more Momentum Transfer Actuators on the flexible portions of the mechanical structure near the points whose position is to be controlled, sensing the position of the flexible portions of the mechanical structure, and, employing one or more feedback loops, to control the Momentum Transfer Actuators to cause the flexible ends of the mechanical structure to follow a desired position as a function of time. The invention also addresses the use of combined actuating and sensing devices that incorporate both a Momentum Transfer Actuator and an inertial sensor to provide acceleration, velocity, and position information to the feedback control system.

FIELD OF THE INVENTION

[0001] This invention related to momentum transfer actuators (MTAs),and, in particular, to the use of momentum transfer actuators to controlthe motion of a flexible mechanical structure.

BACKGROUND OF THE INVENTION

[0002] It is well known in the art to actively damp the vibrationscaused by a machine so as to prevent the coupling thereof to thesurrounding environment. It is also well known in the art to activelydamp unknown vibrations from the environment so that they do not disturba motion sensitive piece of equipment, such as a camera orphotolithography machine.

[0003] Typically, the application of inertial force within a supportmember, under control of a feedback system, is used to cancel out theforce applied to the vibrating side of the support member to achievevibration isolation between the vibrating side of the support member andthe “quiet” side of the support member. The feedback system sensesforces applied to the member and cancels the forces by the applicationof equal and opposite forces. This concept is the basis for a widevariety of active vibration damping schemes, such as those disclosed inU.S. Pat. No. 4,483,425 (Newman), U.S. Pat. No. 4,525,659 (Imahashi, etal.), U.S. Pat. No. 4,929,874 (Mizuno, et al.), U.S. Pat. No. 5,156,370(Silcox, et al.), U.S. Pat. No. 5,327,061 (Gullapalli) and U.S. Pat. No.5,750,897 (Kato). These prior art inventions all make use of anadditional movable mass that is accelerated in such a way as to cancelout the inertial forces that would otherwise be coupled into a baseplate through a coupling member. Examples of both linear motion androtational motion of these inertial proof masses are disclosed in theprior art.

[0004] In many cases improved vibration damping of a mechanicalstructure is achieved by simply adding more material to increase thestiffness of the structure. In such a case, a given disturbance forcewill result in a smaller motion at the flexible end of the mechanicalstructure. However, there are a many economically important applicationsin which other design constraints limit the degree to which increasedmaterial can be added to the system. For example, the mechanical beamsused to construct a space station must be extremely light because theyhave to be lifted into space from Earth's gravity well. When the mass orvolume of the mechanical system is constrained, the flexibility ofmechanical components can often limit the performance of the overallsystem.

[0005] In a typical hard disk drive there is a mechanical arm that holdsthe read/write head at a precise radial position over a desired track ofdata (see FIG. 1). This arm is attached by a rotary bearing at a pointjust outside the outer circumference of the disk. A voice coil actuatoris attached to an extension of this arm on the opposite side of thebearing as the read/write head. Force from the voice coil actuator movesthe back end of the arm, thereby causing the arm to pivot (see FIG. 2).The maximum force that can be generated by the voice coil actuator islimited. Therefore, to achieve a required arm seek time, the rotationalmoment of inertia of the arm, and similarly its total mass, must belimited. For example, note the holes intentionally cut into the armexample shown in FIG. 3 to reduce its mass. Because only a limitedamount of material (mass) can be used in the manufacture of the arm, thearm exhibits an undesired degree of flexibility, and cannot be madearbitrarily stiffer by simply adding more material.

[0006] There are three main disturbances in a hard disk drive thataffect the position of the read/write head relative to the desired datatrack on the disk—actuator bearing hysteresis, windage disturbance, andnon-repeatable disk bearing noise (a.k.a. non-repeatable runout).Actuator bearing hysteresis is a time-dependent nonlinear behaviortypical of mechanical bearings. It typically results in an initial delayin motion resulting from a force applied to the voice coil actuatorfollowed by some overshoot in the desired motion. The windagedisturbance is a non-predictable turbulence force that is applied to thehead. Non-repeatable bearing noise results in the disk itself movingslightly over time in an unpredictable way. Note this unpredictable diskmotion results in a relative position error of the head since it must beover a specific track on the disk for read or write operations to besuccessfully executed.

[0007] Because of the above disturbances, the read/write head may notalways be able to be positioned over the precise spot on the magneticdisk surface that is required for a successful read or write. Therefore,it is desirable that the portion of the arm to which the read/write headis attached be able to be positioned over the correct spot on the diskdespite the disruptive forces described above. In general, it isdesirable that a flexible mechanical structure, such as the arm of ahard disk drive, be made to follow a precise mechanical position ortrack a precise mechanical path (e.g., track out the non-repeatablerunout of the disk) as a function of time, in the presence of disruptiveexternal forces (e.g., the vibration or high frequency oscillationcaused by windage forces).

SUMMARY OF THE INVENTION

[0008] This invention, like the prior art described above, makes use ofinertial force as generated by an (MTA) to affect the motion of aflexible support member. However, the current invention addresses a verydifferent application for this same basic device. Instead of attenuatingor blocking the transmission of a force or torque through a flexiblestructural member, the focus of this invention is the use of feedbackcontrolled inertial force, added to the existing force transmitted inthe flexible structural member, to precisely control the position of aspecific part of a flexible mechanical structure as a function of time.This invention applies to the general case in which it is desired thatthe position of some part of a flexible mechanical structure follow aprecise function of time, and one or more MTAs are used to both canceldisturbance forces and to position the flexible mechanical structure tofollow a desired trajectory.

[0009] Specifically, the invention related to the head of a hard diskdrive, which is typically mounted at the end of a flexible mechanicalstructure. In a typical hard disk drive, the position of the read/writehead relative the desired data track is determined periodically wheneverthe read/write head passes over sector servo marks which are embeddedbetween data tracks. A typical servo burst rate in today's hard diskdrives is on the order of 10 KHz. For feedback loops operating at highfrequencies, the phase delay caused by this periodic sampling of theposition error signal may result in poor stability. Although this couldbe remedied by simply using more frequent servo bursts, doing so wouldtake away surface area that could be used to store data. Therefore, ispreferred to make use of an inertial sensor to augment the sampled dataposition signal available from the servo bursts. One possible use ofthis system would be to interpolate between samples using the inertialsensor.

[0010] Another aspect of this invention is the use of an inertial sensorcombined with an MTA to provide acceleration, velocity, and positioninformation concerning the location of the mechanical structure to whichthe MTA is attached. This information is used in addition to otheracceleration, velocity, or position signals in order to improve theperformance of the overall feedback system.

DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a top view of a the components of a typical disk drive.

[0012]FIG. 2 is a top view of the arm showing the details thereof andthe position of the MTA with respect to the read/write head.

[0013]FIG. 2 is a schematic diagram showing the components of theflexible arm and the MTA.

[0014]FIG. 4 is the top view of an MTA MEMS device

[0015]FIG. 5 is a cross sectional view of a portion of the MTA MEMSdevice of FIG. 4.

[0016]FIG. 5 is a cross sectional view of a second portion of the MTAMEMS device of FIG. 4.

[0017]FIG. 6 is a schematic representation of the device of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The preferred embodiment of the invention is shown schematicallyin FIG. 3. Preferably MTA 40 is a MEMS device construed on a siliconsubstrate. The internal parts of MTA 40 are shown schematically in FIG.3 and include proof mass 44, Y access actuators and suspensioncomponents 42 and encapsulation 46. The actuator may be constructedaccording to methods disclosed in our co-pending U.S. patent applicationSer. No. 09/659,961 entitled “Thin Film MEMS Sensors ImployingElectrical Sensing and Force Feed Back”. One possible implementation ofsuch a device is shown in FIGS. 4-7. FIG. 4 shows a top view of thedevice. It can be seen that proof mass 30, which is suspended by arms 21can easily move in the Y direction. Motion in the X direction isinhibited because of the stiffness in the X direction of arms 21.Further, motion in the Z axis is inhibited by a virtue of electrodes 40and 32 which might be placed above and beneath proof mass 30. FIG. 6shows a cross-section of the device of FIG. 4. It can be seen thatfingers 30 a can be made to move in the Y direction through the use ofelectrostatic force induced by a voltage difference between fingers 30 aand either electrodes 36 a or electrodes 38 a depending upon the desireddirection of movement of proof mass 20. This is shown schematically inFIG. 7 (note that only one set of electrodes and one finger is numberedin FIG. 7, but there are, in actuality, a plurality of each). Throughthe application of voltage differences between electrode 38 a and finger30 a or electrode 36 a and finger 30 a, proof mass 30 can be made tomove in the Y direction while remaining virtually motionless in the Xand Z directions. Also in the preferred embodiment, MTA 40 isencapsulated on the surface of a silicon substrate. This can beaccomplished by manufacturing the device according to the methoddisclosed in our co-pending U.S. patent application Ser. No. 09/583,386entitled “Manufacture of MEMS Structures in Sealed Cavities usingDry-Release MEMS Device Encapsulation.” This is desirable because theelectrostatic forces applied between electrodes 36 a, 38 a and fingers30 a of proof mass 30 would tend to attract dirt particles which couldeventually foul and inhibit the operation of the device.

[0019] Although MTA 40 is shown as a micro-encapsulated MEMS structure,it is also possible to use a macro structure. The advantage of havingMTA 40 constructed as a MEMS device is is its low total mass.

[0020] The preferred embodiment of the invention is described in thecontext of a two stage positioning system of a hard disk drive arm, asshown in FIG. 1. Generally, such a system is composed of an arm 59 whichis free to rotate about bearing 53, read/write head 61 is located at theend of the flexible arm 59, media 49 spins on spindle 51. Flexible arm59 rotates about bearing 53 to move read/write head 61 radially overmedia 49. The arm is driven by voice coil actuators composed of magnet55 and coil 57. The arm is shown in more detail in FIG. 2. In the caseof the preferred embodiment of the invention, voice coil actuator 57acts as a low frequency actuator and MTA 40, positioned near theread/write head as shown in FIG. 2, acts as a high frequency actuator.Because arm 59 is flexible, the frequencies at which the head can beprecisely controlled by the voice coil actuator are limited. Forexample, in a typical hard disk drive this upper limit is on the orderof 1 kHz. MTA 40, attached near read/write head 61, is able to move thehead at much higher frequencies, as the flexibility of the small part ofthe arm between where MTA 40 is mounted and where read/write head 61 ismounted can be very small. By applying high-frequency signals to theinternal actuators of MTA 40, as shown in FIGS. 4-7 and schematically inFIG. 3, the fine position of read/write head 61 can be preciselycontrolled even at high frequencies. The feedback loop controlling thehead position must separate the feedback force into low frequencycomponents, which might be applied by voice coil 57, and high frequencycomponents, which might be applied by MTA 40. The proper application offorces to both the voice coil and the MTA can cause read/write head 61to follow a precise track over the surface of disk 49.

[0021]FIG. 3 shows MTA 40 attached to flexible beam 35 very near topoint 30. Point 30 is the point on the flexible head whose precise pathover disk 49 needs to be controlled. Note that MTA 40 need not beattached exactly at the point shown in FIG. 4. In general, thepositioning accuracy will improve as MTA 40 is attached to mechanicalparts of the structure that are more rigidly connected to point 30 whoseprecise positioning is desired. It is also possible to use multiple MTAsto control multiple flexible parts of more complex mechanicalstructures. In an alternative embodiment of the invention, it ispossible to use one or more macroscopic actuators attached to one ormore parts of the mechanical structures and one or more MTAs attached toone or more flexible portions of the structure.

[0022] In yet another embodiment of the invention, it is possible to usean inertial motion sensor to measure acceleration velocity and positionto provide information on the position of the flexible end of themechanical structure to the overall feed back control algorithm. It iscontemplated that the inertial sensor also be a MEMS device constructedaccording to the methods disclosed in our co-pending patents previouslycited. It is also contemplated that the MTA and the inertial sensor beintegrated on the same substrate and/or constructed into separate or acommon sealed enclosure utilizing the encapsulation manufacturingtechnique disclosed in our co-pending patent application.

[0023] With respect to the feedback control algorithm, it is possible tooperate with a combination of inputs, including inputs from any inertialsensors which are utilized or position inputs derived from the readingof sector server bursts which are written onto the surface of media 49.Sector server bursts can give precise position information to thefeedback control system as to the positioning of read/write head 61relative to the desired track of data on the surface of media 49.

[0024] The present invention has been disclosed in terms of the use ofthe method to counteract vibrations or other mechanically inducedmovements of the flexible arm of the hard disk drive, and to cause theread/write head to follow a precise path around the disk. However, it iscontemplated that the method could be used with any flexible mechanicalstructure to make the structure follow a precise mechanical path. In thecase of the hard disk drive, this precise mechanical path is the onedetermined by non-repeatable motion of the disk. Therefore, theinvention is not meant to be limited to the use of the method in a harddisk drive. Further, the invention is not meant to be limited to theprecise construction of the MTA or inertial sensor. The MTA and inertialsensor may be of many different constructions. The motion of the proofmass maybe rotational or linear and the MTA itself may be a macrostructure and not a MEMS device. Further, when the MTA is a MEMS devicethere are a plethora of possible designs from which to choose. Thedesign shown in FIGS. 5-8 are only one example of a possible design forthe device. Therefore, the scope of the invention is not meant to belimited by the examples used herein but is encompassed by the scope ofthe following claims.

I claim:
 1. A method of controlling the position of a point on aflexible mechanical structure comprising the steps of: providing one ormore momentum transfer actuator mechanically coupled to said point onsaid flexible mechanical structure; and using said momentum transferactuators to move said point such that said point follows a desired pathover time.
 2. The method of claim 1 further comprising the steps of:receiving feedback regarding changes in said position of said point dueto outside forces; and using said momentum transfer actuators tocounteract said outside forces such that said point follows said desiredpath.
 3. The method of claim 1 further comprising the steps of:receiving feedback regarding changes in said desired path; and usingsaid momentum transfer actuators to move said point to accommodate saidchanges in said desired path.
 4. The method of claim 2 wherein saidmomentum transfer actuators are micro-electromechanical devices.
 5. Themethod of claim 4 wherein said momentum transfer actuators areencapsulated in a sealed cavity.
 6. The method of claim 2 wherein saidchanges in said position of said point are changes relative to anotherbody.
 7. The method of claim 6 wherein said flexible mechanicalstructure is the arm of a hard disk drive and further wherein said otherbody is a hard disk media.
 8. The method of claim 5 further comprisingthe step of providing one or more inertial sensors to provide saidfeedback regarding said changes in said position of said point due tooutside forces.
 9. The method of claim 8 wherein said inertial sensorsare micro-electro-mechanical devices.
 10. The method of claim 9 whereinone or more of said inertial sensors and one or more of said momentumtransfer actuators have been manufactured on a common silicon substrate.11. The method of claim 9 wherein said inertial sensors have beenencapsulated in a sealed cavity.
 12. The method of claim 11 wherein oneor more of said inertial sensor and one or more of said momentumtransfer actuators have been encapsulated in a common sealed cavity. 13.The method of claim 2 further comprising the steps of: separating saidoutside forces into high and low frequency components; using saidmomentum transfer actuators to counteract said high frequency componentsof said outside forces; and providing one or more additional actuatorsto counteract low frequency components of said outside forces.
 14. For ahard disk drive having a media, an arm having a read/write headpositioned on one end thereof and a radial actuator for moving the armsubstantially radially along said media, a positioning system for saidread/write head comprising: one or more momentum transfer actuators,coupled to said arm in proximity to said read/write head; and a feedbackcontrol circuit for said momentum transfer actuators, wherein saidfeedback control circuit causes momentum to be transferred from saidmomentum transfer actuators to said arm such that said read/write headfollows a desired path along said media as said media rotates under saidread/write head.
 15. The system of claim 14 wherein said feedbackcontrol circuit receives positioning information from said disk drivespecifying the relative radial position of said read/write head oversaid media.
 16. The system of claim 15 wherein said positioninginformation is derived from sector servo marks read by said read/writehead from said media.
 17. The system of claim 15 wherein said momentumtransfer actuators are micro-electromechanical devices.
 18. The systemof claim 17 wherein said momentum transfer actuators are encapsulated ina sealed cavity.
 19. The system of claim 18 further comprising one ormore inertial sensors, coupled to said arm in proximity to saidread/write head, for providing information regarding forces applied tosaid read/write head which may cause it to deviate from said desiredpath to said feedback control circuit.
 20. The system of claim 19wherein said inertial sensors are micro-electro-mechanical devices. 21.The system of claim 19 wherein one or more of said inertial sensors andone or more of said momentum transfer actuators have been manufacturedon a common silicon substrate.
 22. The system of claim 19 wherein one ormore of said inertial sensors are encapsulated in a sealed cavity. 23.The system of claim 22 wherein one or more of said inertial sensors andone or more of said momentum transfer actuators are encapsulated in acommon sealed cavity.
 24. The system of claim 19 wherein said controlcircuit analyzes said information regarding forces applied to saidread/write head and counteracts high frequency components of said forcesby sending commands to one or more of said momentum transfer actuatorsand low frequency components by sending commands to said radialactuator.
 25. The system of claim 24 wherein said outside forces areselected from a group comprising high frequency oscillations caused bywindage, actuator bearing hysteresis and non-repeatable disk bearingnoise.
 26. A micro-electro-mechanical device comprising: a momentumtransfer actuator; and an inertial sensor; wherein said momentumtransfer actuator and said inertial sensor are manufactured on a commonsilicon substrate.
 27. The device of claim 26 wherein said momentumtransfer actuator and said inertial sensor are encapsulated in a sealedcavity.
 28. The device of claim 27 wherein said momentum transferactuator and said inertial sensor are encapsulated in a common sealedcavity.
 29. The device of claim 26 wherein said momentum transferactuator further comprises: a mass; means for suspending said mass;means for restricting the movement of said mass in one or moredimensions; and means for causing said mass to move in a positive ornegative direction in at least one dimension; and wherein said inertialsensor further comprises: a mass; means for suspending said proof mass;means for restricting the movement of said proof mass in one or moredimensions; and means for sensing the movement of said mass in apositive or negative direction in at least one dimension.